The present invention relates to optical systems, and more particularly to a method and system for providing an isolator/polarization beam combiner.
In fiber optic communication, the optical signal propagates inside an optical fiber. Due to the nature of optical fibers, optical signal attenuation always exists. To overcome this signal attenuation in long distance communication systems, optical signal amplification using an optical amplifier is required. Both Erbium Doped Fiber Amplifier (EDFA) or Raman Amplifier can be utilized as the optical amplifier. In order for the optical signal to propagate a longer distance, an optical amplifier with high output power is preferred.
Pump lasers with high output power are used in order to achieve high output power in high power optical amplifiers. In current technology, the output power of a single pump laser is limited. Higher pump power can be achieved by combining two pump signals having orthogonal polarization states. Furthermore, an optical isolator is required to prevent any back reflected signal from getting back to the pump so that a stabilized combined pump power can be achieved.
FIG. 1 depicts a conventional isolator/polarization beam combiner 10. The conventional isolator/polarization beam combiner includes a dual fiber collimator 16, a Wollaston prism 22, a polarization beam displacer 24, an isolator core 26 and a single fiber collimator 28. The collimators 16 and 28 include pigtails 18 and 32, respectively, lenses 20 and 30, respectively. The lenses 20 and 30 are both GRIN lenses. Furthermore, the optical signals input from the fibers 12 and 14 cross at the face of the GRIN lens 20.
The conventional isolator/polarization beam combiner 10 accepts a first optical signal from fiber 12 and a second optical signal from the fiber 14. The polarizations of the optical signals are typically perpendicular. In addition, the fibers 12 and 14 are typically polarization maintaining (PM) fibers. The dual fiber collimator 16 collimates the optical signals such that the optical signals cross at the face of the GRIN lens 20. The optical signals are then deflected to travel horizontally by the Wollaston prism 22 and combined by the polarization beam displacer 24. The combined optical signal is transmitted by the isolator core 26, which also prevents back reflections from the single fiber collimator 28 from being provided back to the input fibers 12 and 14. The single fiber collimator 28 then focuses the combined signals and outputs this combined signal through the fiber 34.
Although the conventional isolator/polarization beam combiner 10 functions, one of ordinary skill in the art will readily recognize that there are several drawbacks to its operation. First, a large number of discrete components are used: the dual fiber collimator 16, the Wollaston prism 22, the polarization beam displacer 24, the isolator core 26 and the single fiber collimator 28. Because of the large number of components, a large insertion loss is introduced. Moreover, the large number of components makes assembly of the conventional isolator/polarization beam combiner 10 more difficult. In addition, the conventional isolator/polarization beam combiner 10 is expensive, due in part to the use of a large number of optical components and the expense of certain optical components. For example, the GRIN lenses 20 and 30 are expensive to purchase. Furthermore, the conventional isolator/polarization beam combiner 10 has a higher polarization dependent loss because of the separated ordinary and extraordinary beams introduced by the isolator core 26. Thus, the conventional isolator/polarization beam combiner 10 has relatively high losses and is expensive and difficult to assemble.
Accordingly, what is needed is a system and method for combining and isolating two signals with a lower insertion loss, lower PDL and a lower cost. The present invention addresses such a need.
The present invention provides a method and system for providing an isolator/polarization beam combiner. The method and system comprise providing a first collimator, a core and a second collimator. The first collimator collimates a plurality of optical signals traveling in a first direction. The core receives the plurality of optical signals from the first collimator. The core includes a first birefringent wedge, a second birefringent wedge and a rotator between the first and second birefringent wedges. The first collimator, the first birefringent wedge and the rotator transmit the plurality of optical signals such that the plurality of optical signals cross at an interface between the rotator and the second birefringent wedge. The core transmits the plurality of optical signals with a deflection, combines the plurality of optical signals to provide a combined optical signal and prevents a reflected portion of the plurality of optical signals from traveling opposite to the first direction. The second collimator receives and collimates the combined optical signal.
According to the system and method disclosed herein, the present invention provides a low cost isolator/polarization beam combiner.